Part 1. The Field of the Invention
The subject of the present invention is a device enabling the level of a liquid in a tank to be detected, in particular the level of a liquefied gas in a lighter tank. It is to this particular application that detailed reference will be made hereinafter, but it will appear clearly to the individual skilled in the art that the invention extends to any other tank of liquid.
Part 2. Description of the Prior Art
Most gas cigarette lighters have no device giving the user an indication as to the quantity of gas remaining in the tank. Of course, lighters are known whose transparent or translucent bodies enable the level of liquefied gas to be distinguished. However, in addition to the fact that these lighters are generally of mediocre quality, monitoring the gas level is generally difficult because it forces the user to incline the lighter to clearly distinguish the connecting line to the free liquid surface on the wall of the lighter body.
The use has also been proposed of optical devices which, disposed in the bottom of a tank, allow a colored pellet to appear as long as the lighter contains gas, this pellet disappearing when the tank is empty. However, the low luminosity of the signal transmitted and the requirement for the user to make his observation at an angle very close to the axis of the system have caused this type of device to be abandoned.
In different technical fields, another type of device for detecting the level of a liquid in a container is also known. This device has a bar of transparent material, one cone-shaped end of which, with a vertex angle equal to 90.degree., can be immersed or not immersed according to the level of liquid in the container. The refractive index of the bar is chosen, by comparison to that of the liquid, such that, when its end is immersed in the liquid, the incident light rays transmitted by the transparent material refract at the interface separating this material from the liquid and become lost in the liquid, and such that, when this end is not immersed, the incident light rays undergo total double reflection by meeting said interface twice and are sent to the other end of the bar. In the first case, the user perceives a dark part which is, in fact, the bottom of the tank and in the second case he perceives a lighter part. He can thus determine whether the level of liquid is located above or below the end of the transparent bar.
It has also been proposed, with the goal of making remote reading of this type of device possible, that a bundle of light-conducting fibers, hereinafter called optical fibers, be associated with the bar of transparent material. These fibers carry the indicating signal from the measuring point to the observation point. However, due to the low luminosity of their signals, these devices are generally extremely difficult to read.
For this reason, it has been proposed that the paths of the incident, reflected light rays be separated into two separate bundles of optical fibers. Thus, a light source illuminates the transparent bar by a first bundle of optical fibers and, after reflection, the light rays return to the observer via a second bundle of optical fibers.
Although this type of device, due to its good luminosity, proves to be extremely accurate and readable, it can only be used in the case where a light source is available and where space-availability problems are not critical. This is not the case when this device is to be fitted to a tank of small dimensions such as that of a ligher.